Since the mid 1980's it has been clear that dyes (and later other sensitizers such as quantum dots) attached to semiconductor surfaces such as TiO2 efficiently inject charge into the semiconductor conduction band when photoexcited. Since that development, it has been widely recognized that with such heterojunctions, appropriately arranged with a conducting electrode and counter electrode, might enable construction of efficient solar cells. These architectures were termed sensitized solar cells (SSC hereinafter).
The electronic band structure of such an SSC cell is depicted in FIGS. 1A and 1B. In such an SSC cell, light enters from the left side of the page and is transmitted through both the transparent conducting oxide (hereinafter, TCO) anode and also the wide band gap semiconducting oxide (shown as TiO2 in this case, but could be any of a variety of known semiconducting oxides including, without limitation, ZrO2, ZnO, and Nb2O5). The light is adsorbed by a sensitizer molecule dye chemically attached to the TiO2 layer (depicted in FIG. 1A as a wavy line labeled hν). The dye is chosen such that the excited state electron affinity is much less than the ground state. In femtoseconds the dye donates its electron to the TiO2 conduction band. The electron migrates through the TiO2 reaching the anode where it can be used for work. The now reduced dye is regenerated by accepting an electron from a redox shuttle molecule available in solution (typically this involves a 3I−/I3− conversion with the oxidation of two dye molecules. Ultimately the redox shuttle is regenerated at the back electrode. Thus, a wire connecting the TCO and the back electrode sees current flow.
While the solar cell described in FIG. 1A is extremely efficient for each photo-absorbed, useful devices with this architecture have not been made because the absorbance of a single monolayer of sensitizer molecule dye is vanishingly small (of the order of 1 photon in 1000).
Researchers have subsequently attempted to increase the heterojunction absorbance by simply increasing the loading of the sensitizer molecule. This approach was however quickly abandoned when it was realized that although the absorbance increased in such cells, the efficiency per photon dropped precipitously. Ultimately it was demonstrated that only the first monolayer of dye would efficiently inject electrons into the TiO2, all other layers simply lost excitation by generating heat.
The first successful cells based on these heterojunction ideas came from Michael Gratzel's laboratory in Switzerland. Here the absorbance was increased by using a compressed nanoparticle array of TiO2 particles coated with a sensitizer dye as depicted in FIG. 2B. This arrangement meant that photons had to traverse nearly 1500 monolayers of dye rather than one, yet maintained the arrangement of a monolayer of dye on TiO2 and consequently maintained the efficient injection of photoelectrons.
When this architecture was first announced, Gratzel produced cells of ˜10% efficiency and created a sensation. The subsequent flurry of studies has in many ways justified the excitement. Degradation studies for instance have demonstrated that Gratzel photocells have a mean operating life measured in decades. Cost studies have demonstrated that Gratzel photocells are significantly cheaper, made of more abundant material, and are easier to produce than Si based photovoltaics. Thus, if the efficiency of such cells could be raised to about 15%, it is likely that they would totally replace Si cells. This promise has lead to the establishment of a number of companies focused on producing panels of Gratzel photocells and generating many significant public demonstrations (see for instance website http://www.dyesol.com).
Unfortunately, Gratzel photocell efficiencies have increased only marginally (10%->11.2%) in the past decade despite theoretically possible efficiencies approaching 30%. However, the reason for this lack of progress is now clear. Injected photoelectrons have a long and slow path to travel in the TiO2 nanoparticle array in order to find the TCO electrode. During this traverse, which may require seconds, there are many opportunities for losses which in fact do occur at the electrolyte TiO2 interface (see FIG. 2A). As currently constituted, cell efficiencies represent a careful balance between electron drive (voltage loss) and electron recombination (current loss) which is fully optimized. These interfacial losses then result in greatly diminished efficiency relative to theoretical efficiencies.